Flare stack diameter

Figure 6-7. Nomogram for flare stack diameter, knockout drum diameter and length of flame.

Flare stack diameter depends on the Mach number and is estimated by using the following equation ... 

Mach = design Mach number W = flow rate, kg/h P = pressure at flare tip, kPaA d = flare stack diameter, m z = compressibility of the flowing gas T = temperature of the flowing gas, K k = ratio of specific heat MW = molecular weight of the flowing gas... 

Sizing of flare stack Brzustowski and Sommer approach [20] Calculation of flare stack diameter... 

Flare stack diameter is calculated using Equation 4.94 as explained before. 

Total heat released (Equation 4.93) = 1.49 W kW Minimum distance from flame center (Equation 4.92) = 53 m Flare stack diameter (Equation 4.94) = 0.496 m... 

Flare stack diameter (as calculated before) = 0.496 m Tip exit velocity (as calculated before) = 210 m/sec Lower explosive limit of gas (Equation 4.102, Table 4.17) = 0.0397 mol fraction... 

Heal release Flame length. L Flare stack diameter, d Flare tip exit vebcily Flame dstortion Factor. U Concentration parameter, CL Thru perameter, dlR Minimum distance. D Flare stack height, H... 

A secondary seal loop is provided for water withdrawal during major blows when turbulence at the downstream overflow connection to the primary seal loop interferes with normal drainage. Extending the base of the flare stack 3 diameters below the sloped inlet line provides vapor disengaging for the secondary seal leg. The bottom of the stack and inlet line up to 1.5 m above the seal water level are gunite lined for corrosion protection. 

We shall now provide a second example to illustrate step-by-step calculations. In this example a flare stack is estimated to be 80% efficient in combusting HjS off-gas. The total off-gas through the stack is 400,000 kg/hr, of which 7.0 weight percent is H2S. The physieal stack height is 250 m, the stack diameter is 5.5 m, and the stack emission velocity is 18 m/s. The stack emission temperature is 15°C. The meteorological conditions may be described as a bright sunny day with a mean wind speed of 3 m/s. 

For non-smokeless flares (no steam injection) about 30% higher capacity can be allowed [59]. Therefore, the diameter of a non-smokeless flare stack is approximately (0.85) (diameter of the smokeless flare stack). 

However, the vent collection system that served the low-pressure feed tank also served a nearby distillation column that operated at nearly 15 psig and contained a toxic component. Physically the low-pressure tank was on the vent piping between the distillation column and the flare stack. During a distillation column upset, there was an increased flow to the flare. The vent header pressure increased. The vent piping diameter between the distillation column and the flare proved to be too small. Toxic fumes from the higher pressure system column were released from the low-pressure tank emergency tiffing lid. 

During a model study, wind conditions and stack diameter are appropriately scaled down to ensure dymamic similarity. This suffices only the requirements for cold flow conditions. In a burning environment, however, parameters such as fuel pyrolysis time that depends only on fuel chemistry and temperature [66] are also important to be considered. In addition, buoyancy effects are generally neglected in model flares. For all these reasons, the model results must be compared with field test data to validate the correlations developed and develop scaling laws. Due to the unavailability of such data, quantitative scaling laws are yet to be developed. To date, only a few model test results have been compared with field test data. For instance, Schwartz and White [69] compared predictions of radiative emission from various models with field data. Gook et al. [90,91] conducted field-scale... 

A flare stack, particularly the flare burner, must be of a diameter suitable to maintain a stable flame and prevent a blowout should there be a major failure. 

Experiments show that flame blowout occurs when vapor exit velocities are as high as 20 to 30% of the sonic velocity of the stack vapors. These results were obtained with small diameter pipes up to 0.152 inches. There is evidence that higher blowout velocities are attainable with pipes of larger diameter such as flare stacks but in the absence of data on blowout velocities for flare stacks, it is good practice to size flare stacks on a basis of 20% of the sonic velocity as the exit velocity. 

Recent experience shows that stack diameters obtained by means of Figure 6-7 generally agree well with data for smokeless flare stacks. 

From data for nonsmokeless flares, it is also evident that approximately 30% higher capacity can be allowed with this type of flare. Accordingly, to obtain the diameter for nonsmokeless flare stacks, multiply the diameter found in Figure 6-7 by 0.85. 

In general, one should not select a flare stack with a diameter smaller than the diameter of the flare header with which it is associated. 

A knockout drum in a flare system is used to prevent hazards associated with burning liquid droplets escaping from the flare stack. Accordingly, the drum must be of sufficient diameter to produce the desired liquid-vapor separation. 

The operating cost of a 2-ft. diameter flare stack, when purged at the minimum rate, is nearly 1,500 a year (about 4 a day) if fuel gas is valued at 30 per million Btu. 

Replace flare stack with a bigger diameter or taller stack, increase size of relief header install a parallel relief header... 

Another method, given in API RP521 uses the drop-out velocity of a liquid particle as the basis for design the residence time is such that the particle will fall to the liquid surface level within the length of the drum. The minimum particle diameter is assumed to be 150 tm, since flare stacks downstream can cope with particles smaller than this. In the API RP521 method, the drum dimensions are first assumed and then calculations show whether or not these are satisfactory. Often this approach can lead to a tedious trial and error solution. 

Guy cables can be remediation devices to stabilize a dynamic unstable stack or process column, or as a built-in design support mechanism for a tail and narrow diameter stack. A flare stack is more involved than other types of stacks because there are thermal gradients to complicate matters. 

Sizing of a flare stack simple approach Calculation of stack diameter... 

Open flares have a flare tip with no restrietion to flow, the flare tip being the same diameter of the staek. Open flares are effeetively a burner in a tube. Combustion and mixing of air and gas take place above the flare with the flame being fully eombusted outside of the stack. 

Flares are an attempt to deliberately burn the flammable safety relief and/or process vents from a plant. The height of the stack is important to the safety of the surroundings and personnel, and the diameter is important to provide sufficient flow velocity to allow the vapors/ gases to leave the top of the stack at sufficient velocities to pro ide good mixing and dilution after ignition at the flare tip by pilot flames. 

Determine the stack height required to give a heat intensity of 1500 Btu/hr/ft2 at a distance of 410 ft from the base of the flare. The flare diameter is 4 ft, the flare load is 970,000 lb/hr, and the molecular weight of the vapor is 44. 

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